To get HM04 Strength in Pokémon Emerald, you'll need to find the Rock Smash ability and earn the Dynamo badge. You can get Rock Smash in Mauville City, the same location that you can earn the Dynamo badge. once you can use Rock Smash, you can get Strength by clearing the rocks in Rusturf tunnel.

Steps

Part 1

Getting HM06 Rock Smash

1

Head to Mauville City after reaching Slateport City. After defeating Team Aqua in the Oceanic Museum, you can head north to Route 110, which connects Slateport and Mauville. You'll have to walk underneath the bike path for now.

2

Battle your rival. As you make your way up the pedestrian path on route 110, you'll be challenged by your rival to your third rival battle. The Pokémon your rival uses varies depending on who you picked as your starter, but the first two will be Level 18 and the last will be Level 20.

3

Go to the house in the bottom-right corner of Mauville. It has a pink roof. You'll find a solitary man inside the house.

4

Talk to the man inside to get HM06 Rock Smash. After explaining its usefulness, the man will give you HM06 Rock Smash.

5

Equip Rock Smash on a Pokemon. Teach Rock Smash to a Pokémon now if you want to use it in battle against Wattson. You won't be able to use it outside of battle until you beat the Mauville City Gym and earn the Dynamo Badge.[1]

Part 2

Beating the Mauville City Gym

1

Put your party together. You'll be facing primarily Electric-type Pokemon, so you'll want to bring some Ground-type Pokemon, which are immune to electric attacks. Geodude and Nincada can be very useful Pokémon to use as your primary fighters. Makuhita and Torchic are good for dealing with some of the trainers in the gym before the big fight.[2]

If you chose Mudkip as your starter, evolving it into Marshtomp at Level 16 can help a lot for this gym.

Avoid bringing Water and Flying-type Pokemon, as they are very weak to Electric moves. The exception to this are Pokémon that are Ground-type as well, such as a Flying/Ground

2

Fight Wally outside the gym. Before you can enter the gym, Wally will approach you and ask you for a fight. He just has a single Level 16 Ralts, which shouldn't pose too much trouble.

3

Make your way through the gym. You can skip most of the trainers if you'd like, but it's recommended that you fight as many as possible for the rewards and experience. To get to Wattson, activate the switch in the southwest corner of the room.

4

Defeat Wattson. Wattson will have a Level 20 Voltorb, a Level 22 Magneton, a Level 20 Electrike, and a Level 24 Manectric. The Voltorb's Self Destruct move will likely cause whatever Pokémon is fighting it to faint, so be prepared. Use Ground moves as much as possible to deal lots of damage to the Magneton and other Pokemon.

Once you defeat Wattson, you'll receive the Dynamo badge. This will allow you to use Rock Smash outside of battle.

Part 3

Finding HM04 Strength

1

Head west on Route 117. Now that you have the Dynamo badge, you can use Rock Smash to clear the rocks in Rusturf Tunnel. Take Route 117 west out of Mauville City to reach Verdanturf Town.

2

Head north through the Rusturf Tunnel. Once you're in Verdanturf Town, enter the tunnel in the rock face to the north.

3

Use Rock Smash on the rocks blocking the path. Head north in the tunnel, and then around the bend to the west. You'll come across two rocks blocking the path, Approach the rocks and use Rock Smash when prompted to clear the route.

You'll need to have the Pokémon that knows Rock Smash in your active party.

4

Get HM04 Strength from Wanda's boyfriend. Once you clear the rocks, Wanda's boyfriend will thank you for reuniting them. He'll give you HM04 Strength as a reward.

5

Teach Strength to a Pokemon. You can teach the move to a Pokémon immediately, but you won't be able to use it outside of battle until you beat the next gym in Lavaridge Town and earn the Heat Badge.[3]